1. Field of the Invention
The present invention relates to an apparatus for detecting deterioration of a catalyst of an internal combustion engine in which a main air/fuel ratio sensor is disposed in each exhaust system of a plurality of cylinder banks, and a sub air/fuel ratio sensor is disposed in the lower stream from a catalyst provided in a collecting exhaust pipe of the cylinders, the apparatus being arranged to detect the deterioration of the catalyst in accordance with the results of detections performed by the main air/fuel ratio sensors and the sub air/fuel ratio sensor.
2. Description of the Prior Art
Hitherto, an internal combustion engine has been provided with main air/fuel ratio sensors in the upper stream from the ternary catalyst to feedback-control the air/fuel ratio in order to improve the purification efficiency of the ternary catalyst disposed in the exhaust system thereof and to improve the fuel consumption efficiency. However, the main air/fuel ratio sensor disposed in the upper stream from the ternary catalyst easily deteriorates due to heat. Therefore, a so-called double sensor system is constituted by additionally providing a sub air/fuel ratio sensor in the lower stream from the ternary catalyst so as to cause the sub air/fuel ratio sensor in the lower stream to supervise the control of the main air/fuel ratio sensor in the upper stream, so that a correction is performed.
In an internal combustion engine of the aforesaid type, the deterioration of the catalyst is discriminated in accordance with the results of the detection operations performed by the main air/fuel ratio sensors and the sub air/fuel ratio sensor. For example, there have been a known method which employs the frequency ratio by using the outputs from the two air/fuel ratio sensors as parameters (refer to Japanese Utility Model Laid-Open No. 63-128221), a method which employs the amplitude ratio (refer to U.S. Pat. No. 3,962,866), and a method which employs the area ratio (refer to SAE910561).
There is sometimes also employed the double sensor system in a V-type engine having two cylinder banks and two exhaust manifolds. The aforesaid V-type engine is, due to this structural requirement, constituted in such a manner that a catalyst is disposed in a collecting exhaust pipe which combines the two exhaust systems and a sub air/fuel ratio sensor is disposed in the lower stream from the catalyst in order to decrease the number of the sub air/fuel ratio sensor disposed in the lower stream. Furthermore, supervision of the air/fuel ratio feedback control of each cylinder bank performed by the two main air/fuel ratio sensors disposed in the upper stream is carried out by one sub air/fuel ratio sensor disposed in the lower stream.
Hence, in the V-type engine, the deterioration of the aforesaid catalyst has been discriminated in accordance with the results of the detection performed by either of the main air/fuel ratio sensors disposed in the upper stream and the sub air/fuel ratio sensor disposed in the lower stream.
Therefore, in the V-type engine, the sub air/fuel ratio sensor disposed in the lower stream detects the composition of the mixture gases exhausted from the two exhaust systems. In a case where the control phase of the first bank and that of the second bank coincide with each other as shown in (A) of FIG. 20, the same density gases, that is, two rich gases or two lean gases are mixed with each other in the collecting exhaust pipe in such a manner that if a rich gas is exhausted from one of the two exhaust systems, also a rich gas is exhausted from the other exhaust system, and if a lean gas is exhausted from one of the exhaust systems, also a lean gas is exhausted from the other system. Therefore, the waveform can be measured by the sub air/fuel ratio sensor disposed in the lower stream.
However, the fact that the air/fuel ratio feedback control is performed for each bank will cause a problem to arise in that the rich gas and the lean gas air mixed with each other to become a gas having an air/fuel ratio of .lambda.=1 in the collecting exhaust pipe if the control phases of the two bank become different from each other and the exhaust air/fuel ratios become different in the opposite directions as shown in (B) of FIG. 20. If no amplitude is attained in the waveforms detected by the sub air/fuel ratio sensor as described above, the discrimination whether or not the catalyst has deteriorated cannot be made.